1. Field of the Invention
The present invention relates to an optical low pass filter to be used in an optical system for a single tube or double tube color television camera and more particularly to an optical filter selectively capable of both phase and amplitude modulation.
2. Description of the Prior Art
There have been proposed and known various types of optical low pass filters in the prior art which are used in single or double tube color television cameras for eliminating cross-talk between luminance and chrominance signals. One example is a phase grating filter in which transparent optical elements such as laminae, dots, strips or the like of given size are regularly or random disposed on a transparent substrate to cause phase retardation, see for example, U.S. Pat. No. 2,733,291, U.S. Pat. No. 3,681,519 and U.S. Pat. No. 3,756,695. These type of optical filters can provide a signal low pass effect without a substantial loss of light during the transmission, and further desired OTF (optical transfer function) characteristics can be obtained in these optical filters by selecting the size of the optical elements. The OTF characteristic obtained through these phase grating low pass filters, however, varies depending on the wavelength of light passing there through because the OTF characteristic is a function of optical thickness of the optical elements which is related to the wavelength.
The prior art has also used amplitude modulated optical filters such as shown in U.S. Pat. No. 3,566,013 which discloses an amplitude type low pass filter having alternative and parallel strips of different transmissivity. According to this type of optical filter, the OTF characteristics will not vary in accordance with the wavelength. In addition, the OTF value can be designed to become zero at a predetermined cutoff spatial frequency. This latter property may be of advantage for the primary purpose preventing interference and false signals between the chrominance and luminous signals. However, it sometimes is desirable or necessary that the OTF does not become zero for all wavelengths, for instance, as pointed out in U.S. Pat. No. 3,911,479. Further, an amplitude type of filter has the disadvantage of causing light loss when the light passes there through.
Since the television industry is extremely cost competitive, there is a continual desire to improve the video encoding performance while reducing cost. Thus, any optical element that can perform more than one task would be highly desirable.